Many birds travel in flocks, sometimes migrating over thousands of miles. But how do the birds decide who will lead the way? Researchers reporting in the Cell Press journal Current Biology on November 25 now have some new insight based on studies in homing pigeons. For pigeons, it seems, leadership is largely a question of speed.
"This changes our understanding of how the flocks are structured and why flocks of this species have consistent leadership hierarchies," says Dora Biro of the University of Oxford.
Previous studies had shown that flock leadership is unrelated to social dominance. Giving followers extra training flights doesn't promote them to a position of leadership, either. The new findings offer an elegantly simple explanation for the phenomenon of leadership in birds, with important implications for how spatial knowledge is generated and retained in navigating flocks.
While many birds travel in flocks, homing pigeons are domestic and more easily studied than most. "We can control the composition of the flocks and the starting points for their homeward journeys," says Benjamin Pettit, the first author of the new study. "We also have a good understanding of their individual spatial cognition, in particular how their homing routes develop over repeated flights in the same area."
Recent developments in sensor technology also make it possible to explore with exquisite precision how pigeon flocks are coordinated. The latest GPS loggers allow the researchers to track not only the birds' overall routes, but also the sub-second time delays with which they react to each other while flying as a flock.
In the new study, the researchers compared pigeons' relative influence over flock direction to their solo flight characteristics. Their studies showed that a pigeon's degree of leadership could be predicted by its speed in earlier flights.
In solo flights, leaders were no better than followers in forging a straight path. In other words, they didn't excel in navigation ability, at least not at first. When the researchers tested the birds individually after a series of flock flights, however, they found that leaders had learned straighter homing routes than followers.
"Some birds are naturally faster and consistently get to the front, where they end up doing more of the navigation, which means on future flights they know the way better," Biro says. "You can compare this to a 'passenger-driver'-like effect: drivers in a car have to pay attention while passengers are often unable to recall the route they were driven along, especially if they remained passive in the navigation process."
While it's often tempting to take an anthropocentric view of leadership, the findings come as a reminder that leadership can arise as an unavoidable consequence of individual differences within a population. A very simple, self-organizing mechanism--such as that based on variation in speed--is sufficient for leadership to arise. In addition, the new findings offer a mechanism through which leaders can improve in their roles over time, making increasingly better decisions that others can follow.
"Our findings broaden the range of species and situations in which we would expect to see leadership and explain how leadership and competence can naturally come to correlate," Pettit says.
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